The present invention relates to a method of producing perlite filter aids from particulate perlite fines and the resulting product.
Particulate perlite fines are the by-products of the production of cellular perlite particles. The mineral perlite is an igneous rock, generally tan or gray, that is generally comprised of from 65 to 70 weight percent silica, 12 to 16 weight percent alumina, zero to 6 weight percent water with small amounts of the oxides of sodium, potassium, calcium and magnesium.
The presence of the water provides the mineral with a unique property when it is exposed to high temperatures. In the range of from 1400.degree. to 2000.degree. F., the bulk material is softened and the volatilization and release of the vaporized water causes a sudden expansion of the mineral to several times its original size. The expanded product may have a density in the range of from 2 to 10 lbs./cu ft.
The expansion heat treatment is not always uniform and the nature of the final product depends on the water content and particle size of the mineral, the temperature of the heat treatment and the rate of heating. If the pre-expanded mineral is heated too slowly, the combined water is removed from the mineral through pores in the mineral leaving a slightly porous but relatively dense product. If the rate of heating is excessive, then the expansion of the water may be so violent as to shatter the perlite particle forming numerous fines.
There are many uses for the fines produced by the perlite expansion process, one of which is termed a filter aid. A filter aid is a particulate material, inert to the liquid to be filtered, used to remove finely divided colloidal suspensoids from liquids. The material comprising the filter aid is deposited as a layer on a conventional filter medium and a good filter aid will retain a large proportion of the suspended material in the filtrate while allowing a substantial flow rate of filtrate through the filter.
One problem associated with the use of perlite fines as filter aids is the fact that extremely small perlite particles reduce the flow rate of filtrate to a degree that the filtration process utilizing such materials is uneconomically slow. An improvement in the flow characteristics of perlite fines is accomplished by agglomerating the particles to form larger particles having more favorable filtration characteristics.
Typical of the prior art attempts to agglomerate perlite fines to form an improved filter aid is the disclosure of U.S. Pat. No. 3,235,635 which teaches an agglomeration technique comprising the spraying of an atomized liquid on suspended perlite fines. The liquid coalesces the fine particles of perlite and the resulting agglomerates are subjected to a calcining step. The primary disadvantage of this method is that the product has a low attrition resistance unless the time/temperature combination of the calcining process is substantial enough to promote significant amounts of diffusion bonding the particles in the agglomerate. The energy requirements for such a strengthening of the product are high and normally some type of flux or binder is used to bond the particles into an agglomerate. This prior art reference also teaches the use of soda ash admixed in water solution with the perlite fines to improve the sintering characteristics of the particles in the agglomerate. Both the addition of a coalescing liquid and additions of solid flux in a liquid medium have a severe shortcoming in that large amounts of energy are required to remove the liquids from the initial mixture to form the final product.
The present invention provides an agglomerated perlite product well adapted for use as a filter aid without the large energy requirements associated with the removal of liquids during heating operations. In addition, a filter aid produced by the present invention has excellent filtering properties with relatively high permeability flow rate ratios and low cake densities. The agglomerated product also exhibits excellent strength as measured by its resistance to attrition.